New plaster is extremely porous. Dulux describe it as so permeable that it will soak up any moisture quickly, and that is exactly the problem a mist coat solves.
What goes wrong without one
Paint a fresh plaster wall with undiluted emulsion and the plaster pulls the water out of the paint almost on contact. The pigment and binder are left stranded on the surface without penetrating, so instead of bonding, the paint sits on top as a film.
The visible result is patchiness, with some areas flat and matt and others slightly sheened, because the wall absorbed unevenly. The structural result is worse: the film has nothing to grip. It will craze, flake, or lift off in sheets when you next pull tape off the wall. Dulux put it plainly, warning that emulsion applied to wet plaster is likely to crack and peel off.
The ratio
Dulux's own guidance is ten parts paint to one part water. You will find plenty of trade forums recommending anything from three to one upward, and painters genuinely disagree about this.
The disagreement is mostly about how absorbent the particular wall is, which varies with the plaster used and how dry it is. A pragmatic approach is to start at the manufacturer's ratio, brush a test area, and thin further if the paint is dragging rather than flowing.
Two rules matter more than the exact number. Use a cheap contract or trade matt white emulsion, not a vinyl silk and not a paint containing additives, because vinyl forms a film that defeats the purpose. And never use a self-priming or one-coat paint as a mist coat, as those are formulated to sit on the surface, which is the opposite of what you want.
If mixing paint and water in a bucket sounds like a job you would rather skip, Dulux sell a ready-made plaster sealer that does the same thing without the guesswork.
How to apply it
Work it in rather than laying it on. The mist coat is doing a priming job, so the goal is penetration, not coverage. Expect it to look thin, streaky and unimpressive when dry, because it is meant to.
It will dry fast, often within an hour or two, because the wall is drinking it. Some walls, particularly very absorbent ones, benefit from a second mist coat before the top coats go on. If the first coat vanished into the wall and left almost no trace, apply another.
After that, two full-strength top coats will give you a proper finish. Plan for four coats in total on new plaster and you will not be disappointed.
The mistake that costs the most
A mist coat is not a substitute for letting the plaster dry. It is what you do after the plaster is dry, not a way of getting on with it sooner.
Applying even a thinned coat to plaster that is still patchy brown seals moisture into the wall. You will get blistering, brown staining where salts are drawn to the surface, or a bloom of white efflorescence, and the only fix is to strip it back and start again once the wall has properly dried.
The whole point is to prime a dry, porous surface so that paint can bond to it. Get the order wrong and the mist coat becomes part of the problem rather than the solution.